CT (computer tomography) scans an object using particle rays which pass through an object, such as radiation rays, to construct an image within the object. Three-dimensional volume data which are obtained with the CT may be used in understanding a shape or a construction within the object, so that the CT is widely used in non-destructive inspections of the object, medicine, etc. For example, an arbitrary cross-sectional image of the object, or a perspective projection image of the object from an arbitrary viewpoint is reconstructed from the three-dimensional volume data to cause the state within the object to be visualized.
In radiation treatment, image comparison is performed between DRRs (digital reconstructed radiographs) reconstructed from the three-dimensional volume data generated by a device for CT and X-ray images acquired by capturing a patient immediately before the treatment. Alignment between a lesion and a radiation irradiation region is performed based on results of the image comparison. The radiation treatment is a treatment method in which the radiation rays are irradiated onto the lesion of the patient to destruct the tissue of the lesion. Therefore, an occurrence of misalignment between the lesion and the radiation irradiation region causes the tissue of the lesion to remain. To accurately perform the alignment between the lesion and the radiation irradiation region, three-dimensional volume data of the patient are acquired by the CT before the treatment and the position of the lesion is detected three-dimensionally. Moreover, based on the position of the lesion that is detected three-dimensionally, a treatment plan is established. The treatment plan specifies the radiation intensity and the direction of irradiating the radiation rays efficiently to the lesion.
Moreover, in the treatment, when the radiation rays are irradiated in accordance with the treatment plan, it is necessary to align the position of the patient when the treatment plan is established and the position of the patient when the treatment is carried out. Therefore, image comparison is performed between the X-ray images acquired by X-ray capturing immediately before the treatment and a plurality of DRRs reconstructed from the three-dimensional volume data used in the treatment plan, and the DRR which most resembles the X-ray images is searched. Then, the misalignment is calculated between the position of the patient when the treatment plan is established and the position of the patient immediately before the treatment, and a bed on which a patient lies is moved based on the calculated misalignment. This alignment is performed in three-dimensional space, so that the image comparison is performed between each of the X-ray images shot from multiple directions and the DRRs.
The time at which the X-ray images are shot and the time at which the three-dimensional volume data to be a source for the DRRs may differ, causing misalignment in a posture, etc., of the patient. For example, the opening degree of the jaw of the patient may differ, causing misalignment in a joint. Moreover, misalignment may occur since it is likely for deformation to occur in a soft tissue in the body, etc. The misalignment in the joint and the shape may be a factor which adversely affects the image matching. Then, one or both of a region on an image used in the image comparison and a region on an image not used in the image comparison may be set by a user to cause the precision of the alignment to be improved. A region set by the user herein is referred to as ROI (Region of Interest).
The ROI is possibly set by the user in the X-ray images, not in the large number of DRRs that are reconstructed at the time of alignment. However, when the ROI is set in the X-ray images, the ROI needs to be set for each time of treatment in radiation treatment performed over multiple times. On the other hand, if the ROI is set on the three-dimensional volume data, setting the ROI for each time of treatment may be omitted, so it is desirable to set the ROI on the three-dimensional volume data. Moreover, the three-dimensional volume data acquired in the CT show the organs of the patient, making it possible to directly set the lesion or an affected area as the ROI.
Setting the ROI in the three-dimensional volume data may be performed by specifying the ROI on a cross-sectional image to thereby set the ROI in the three-dimensional volume data; however, the ROI needs to be specified on the large number of cross-sectional images to set a three-dimensional region as the ROI, requiring expense in time and effort.